1. Field of the Invention
The present invention relates to a hot isostatic pressing method and a hot isostatic pressing apparatus for, for example, diffusion bonding of different materials in an inert gas atmosphere held at a high temperature and a high pressure.
2. Description of the Related Art
The hot isostatic pressing method (hereinafter may be referred to as “HIP method”) has proved to be effective in improving mechanical properties, diminishing variations in properties and improving the yield and is in wide industrial use as a technique wherein a workpiece is treated at a high temperature of not lower than its recrystallization temperature in a high pressure gas atmosphere of several 10 to several 100 MPa to eliminate pores remaining in a cast product or a sintered product such as a ceramic product.
A conventional hot isostatic pressing apparatus (hereinafter may be referred to as “HIP apparatus”) used for the aforesaid purpose has such a structure as shown in FIG. 18 wherein an electric furnace of a resistance wire heating type is accommodated in the interior of a vertical, cylindrical high pressure vessel 101. In the interior of the high pressure vessel, heaters 102 of a resistance wire heating type are disposed vertically in plural stages so as to surround a treatment chamber. This is for the following reason. A temperature distribution such that an upper portion is high in temperature and a lower portion is low in temperature is apt to occur due to a vigorous natural convection of a high pressure gas and therefore an isothermal condition is to be ensured by heating throughout the whole in the vertical direction. Further, a natural convection of gas can contribute to the phenomenon that the heat for heating and raising the temperature of a treatment chamber 103 is dissipated too much to the exterior of the system. In order that such a phenomenon can be suppressed efficiently, a structure of the treatment chamber 103 and the heaters 102 being enclosed by a bottomed cylindrical heat insulating structure 104 is popular as an optimum method. The heat having passed through the heat insulating structure 104 and transferred to the high pressure vessel 101 is removed by cooling water flowing in a water-cooling jacket portion 105.
According to the ordinary treatment performed in the HIP method, first evacuation and gas purging are firstly performed for removing air from the interior of the HIP apparatus, followed by raising the temperature and pressure, secondly holding the temperature and pressure in predetermined conditions and finally decreasing the temperature and pressure for taking out the treated product. In the HIP method, the cycle time required for all of these steps is long, so that the treatment capacity of the high pressure vessel which is expensive is deteriorated, resulting in increase of the treatment cost. Thus, shortening of the cycle time has been an important subject in industrial production in order to attain a wide spread of the HIP method.
Particularly, in the cycle time, the proportion of the time required for the cooling step is long because cooling is slow and this point poses a problem. A rapid cooling technique as a technique for remedying this drawback has made a rapid progress and at present there generally is performed rapid cooling in an HIP apparatus having a treatment chamber exceeding 1 m in diameter.
As rapid cooling methods there have been proposed a method which utilizes a natural convection created by a difference in gas density (U.S. Pat. No. 4,217,087) and a method wherein a fan or a pump are installed in the interior of a high pressure vessel to produce a forced convection in addition to the natural convection of gas (Japanese Utility Model Publication No. Hei 3-34638).
In these methods, however, there is a fear that in the interior of the treatment chamber the upper side may become higher in temperature, resulting in easy occurrence of a temperature distribution. In an effort to solve this problem there has been proposed a method wherein two fans capable of being controlled each independently are provided, thereby permitting soaking in the interior of the treatment chamber and cooling speed control to be done each independently (U.S. Pat. No. 6,250,907).